According to the related art, an air-conditioning system is more and more widely used, and more and more generally applicable to various workplaces, residences and etc. Also, people have higher and higher requirements for comfort of air conditioners, and especially pay more and more attention to the resulted comfort when heating at low outdoor temperature. However, despite the development of air-conditioning technology, it is inevitable for most air conditioners to encounter a significant reduction of heating effect along with the decrease of the outdoor temperature while heating at low temperature or preparing hot water, thus resulting in a low indoor temperature or a low water-outlet temperature, which degrades the comfort of using air conditioners.